Help with choosing SIP client.

I am using fly number and usually I have it forwarded to my locally mobile number but when I receive calls it is 4cts a minute, would rather it be free if possible.

So I've been informed of a new option which allows me to use a SIP client, they recommended Acrobits saying the plus side was I can receive calls even when the app is not open, but I would rather have something that is free, anyone knows if there's anything out there that would patch calls thru with the app off?

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